Electro-optic modulators using nematic liquid crystals are extensively used as displays, spatial light modulators, and other specialized optical elements. Most of these use a twisted nematic liquid crystal which is aligned at opposite sides of the cell in perpendicular planar directions. The average direction in which the long axes of the liquid-crystal molecules point is called the director. Thus, the directors at the two inner surfaces of the cell are parallel to the surfaces but perpendicular to each other. In between the two sides, absent other effects, the director slowly twists through 90.degree.. The liquid crystal is birefringent; that is, it has a refractive index difference .DELTA.n between the directions parallel and perpendicular to the director. If EQU .DELTA.n.multidot.d&gt;&gt;.lambda., (1)
where d is the thickness of the cell and .lambda. is the wavelength of light, then the linear polarization of light traversing the cell follows the twist; that is, the twisted nematic waveguides the light. In the usual configuration, sheet polarizers are placed on both sides of the cell in correspondence with the alignment direction, that is, in perpendicular orientations. Thus, the undistorted, slowly twisted nematic liquid crystal waveguides the light and therefore rotates the linear polarization of the light so whatever passes through the first polarizer also passes through the second polarizer. However, if a voltage is applied across the cell, the liquid-crystal molecules are reoriented due to a dielectric torque, and the molecules become aligned with the field, thus destroying the slow twist which produced the waveguiding.
On the other hand, several classes of ferroelectric liquid-crystal devices have been proposed. See, for example, the review articles by Patel et al., "Ferroelectric liquid crystal devices," Proceedings of SPIE, volume 613, 1986, pp. 130-134 and "Properties and applications of ferroelectric liquid crystals", Optical Engineering, volume 26, 1987, pp. 373-384. A ferroelectric liquid crystal is one in which the liquid-crystal molecule is chiral and forms smectic layers. In such a system, a polarization develops having a vector direction within the plane of the layer and perpendicular to the director. See Patel, "Electro-optics of ferroelectric liquid crystals," Proceedings of SPIE, volume 567, 1985, pp. 81-85. In most ferroelectric liquid-crystal devices, such as disclosed by Isogai et al. in U.S. Pat. No. 4,586,791, the material is confined between two glass plates such that the director lies in the surface plane at the surfaces. In this geometry, if the cell surfaces are rubbed in a particular direction, then the smectic layers develop in the smectic A phase such that the smectic planar layers are normal to the surface and the layer normal is parallel to the rubbing direction. When this material is cooled into the smectic C phase, two degenerate states are possible, due to the fact that the tilt angle in the smectic C* phase has a non-zero value of the tilt angle (defined with respect to the layer normal) and it could be positive or negative. Thus, in presence of an electric field applied across the two surfaces, the spontaneous polarization couples with the applied electric field and forces the molecules to reorient such that the molecules are collinear throughout the cell. The orientation of the molecules is such that it lies in the surface plane and at an angle equal to the tilt angle with respect to the rubbing axis. The direction of the applied electric field determines whether the molecules tilt at positive or negative tilt angles. When the prior-art smectic C* liquid crystals are so formed, the low temperature tilt angle is generally about .+-.22.degree. but it then decreases with temperature to a 0.degree. tilt angle, at which point there is a transition to the cholesteric phase.
In the case of materials that don't possess a smectic A phase, the material behaves quite differently. In this case the materials usually has a higher temperature cholesteric phase, that is, a chiral liquid crystal that does not possess a layered structure. Thus, when a material with a higher temperature cholesteric phase is introduced into a cell which has been rubbed in a particular direction, the liquid crystal aligns with the director parallel to the rubbing axis in the cholesteric phase. This material when further cooled into the smectic C phase, the director orientation is essentially preserved, but the molecules form a layered structures with the layer normal at an angle equal to the tilt angle to the rubbing axis. The tilt angle in these materials is generally about 45.degree. and fairly independent of temperature.